1. Field of the Invention
The present invention relates to a power generation method using a thermoelectric element whereby electric energy is directly obtained from thermal energy. The present invention also relates to a thermoelectric element that directly converts thermal energy into electric energy, a fabrication method of the thermoelectric element, and a thermoelectric device.
2. Related Background Art
Thermoelectric generation is a technology for directly converting thermal energy into electric energy using the Seebeck effect, i.e. a phenomenon in which an electromotive force is generated in proportion to a temperature difference created between opposite ends of a substance. This technology has been used practically, for example, for a remote area power supply, a space power supply, and a military power supply.
Conventional thermoelectric elements typically have a configuration known as the “π-type structure,” in which a p-type semiconductor and an n-type semiconductor, having carriers of opposite signs, are combined with each other thermally in parallel and electrically in series.
Generally, the performance of a thermoelectric material used for the thermoelectric element is evaluated by a figure of merit Z, or a figure of merit ZT that is obtained by multiplying Z by absolute temperature T to be non-dimensionalized. ZT can be expressed as ZT=S2/ρκ, where S is a Seebeck coefficient, ρ is electrical resistivity, and κ is thermal conductivity of the thermoelectric material. The figure S2/ρ, which takes into account only the Seebeck coefficient S and electrical resistivity ρ, is called a power factor (output factor) that is used as a measure of evaluating the generating performance of the thermoelectric material under a constant temperature difference.
The thermoelectric material Bi2Te3 that has been put to practical applications until today has a relatively high thermoelectric performance with a ZT of about 1 and a power factor of 40 to 50 μW/(cm·K2). However, it has been difficult to retain a high thermoelectric performance when the material is used in an element of the π-type structure. As such, the performance of this material is not sufficient for use in a wide range of practical applications.
Meanwhile, as an element having a structure other than the π-type structure, an element has long been proposed that takes advantage of the anisotropy of thermoelectric properties of layered structures found in nature or artificially produced (Thermoelectrics Handbook, Chapter 45 “Anisotropic Thermoelements”, CRC Press (2006): Reference 1.) However, as taught in Reference 1, improvement of ZT in this type of element is difficult to achieve, and for this reason developments have been made primarily for measurement applications such as infrared sensors, instead of applications concerning thermoelectric generation.
A thermoelectric material having a similar structure is disclosed in JP 6(1994)-310766 A (Reference 2), in which a material having thermoelectric properties as represented by Fe—Si materials, and an insulating material having a thickness of 100 nm or less as represented by SiO2 are alternately arranged in stripes on a substrate. According to Reference 2, while the material having this kind of microstructure is able to improve in Seebeck coefficient S compared with the sole use of the Fe—Si material having thermoelectric properties, inclusion of the insulating material increases electrical resistivity ρ. This leads an element using the material increase its internal resistance with the result that the power is reduced on the contrary.
There is another type of thermoelectric material having a layered structure, as disclosed in, for example, International Publication pamphlet 00/076006 (Reference 3) that teaches a material having a laminar body of a semimetal, a metal, or a synthetic resin. This material is intended for the structure in which, as in the conventional π-type structure, a temperature difference is created along the direction in which the constituting layers of the laminar body are laminated, and power is extracted via a pair of electrodes disposed face to face in this direction. The element disclosed in Reference 3, therefore, fundamentally differs from the element disclosed in Reference 1.